The present invention relates to medical imaging with a multidimensional array. In particular, medical imaging at selected frequency bands with a multidimensional array is provided.
Imaging at particular frequency bands may provide increased resolution and signals associated with less clutter or noise. For example, signals at a second harmonic of a fundamental transmit frequency have increased resolution and decreased clutter level as compared to signals at the fundamental frequency band. One way to obtain information at a desired frequency band is to apply relative apodization, delay or phase patterns across the elements of a one-dimensional transmit aperture or across scan lines for a two-dimensional plane. By combining information associated with different phases, delays or apodizations, information at desired frequency bands may be isolated from information of undesired frequency bands. For example, as disclosed in U.S. Pat. No. 6,193,659, every other element of a transmit aperture transmits a waveform with a first phase. The other elements transmit the waveform 180 degrees out of phase with the first waveform. The waveforms sum in the acoustic domain, maintaining information at a second harmonic frequency band while canceling out information in the transmit signal at the same or fundamental frequency band. As yet another example, alternating scan lines are associated with different phases, such as an alternating periodic pattern of a 180 degrees phase shifts between adjacent scan lines. By combining information associated with two or more scan lines, various frequency bands may be cancelled while isolating information at other frequency bands. However, applying a periodic pattern across the transmit aperture may increase grating lobe levels. Increased grating lobe levels contribute clutter to an image. Applying a periodic pattern across scanlines may use overlaps between scan lines, so may reduce frame rate.